Semiconductor laser diode assemblies or diode “bars” are commonly used for many applications, including to optically excite or “pump” the gain media of other lasers. Such diode bars typically have a series of light emitting laser diodes aligned along a common substrate at regular intervals, forming a row or “stripe” of diode emitters.
FIG. 1 is a perspective view of a prior art laser diode bar 100. In this example, the diode bar includes ten diode emitters 102. Each diode bar typically has a width 110 that is approximately 150 microns along an axis 116 that is perpendicular to an axis 112 of the diode bar stripe. The diode bar 100 emits a combined output from the combined emitting areas of the individual diode emitters 102. Each diode emitter 102 typically has an area with a width 108 and length 106 that are, for example, one micron and 100 microns, respectively.
Output beams 104 produced by the individual diode emitters 102 along an output axis 114 have a relatively broad angular divergence in one direction or axis 118 and a smaller degree of divergence in the orthogonal direction or axis 120. This pronounced difference in angular divergence characterizes the output of edge-emitting laser diodes. These axes are often referred to as the “fast” and “slow” axes, 118 and 120, respectively. For example, the angular divergence θ1 of an output beam 104 may be approximately 30° along the fast axis 118 and the angular divergence θ2 of the output beam may be approximately 10° along the slow axis 120. This difference in angular divergence produces an output 104 having an elliptical distribution as shown in FIG. 1. For the angular divergence angles given, a ratio of the angular divergences, or aspect ratio, of 3:1 is produced. Typical aspect ratios for diode lasers may range from, for example, 2:1 to 10:1.
While laser diodes have a relatively high electrical efficiency compared to other types of lasers, an individual laser diode typically operates at a relatively low-power. Laser diode bars have output powers that are scaled according to how many individual laser emitters are included within the particular diode bar. One limitation on the output power of laser diodes and diode bars is the generation of excess heat during operation of the diodes. Cooling means have been used to remove heat in attempts to increase power from a given diode bar.
FIG. 2 is a perspective view of the prior art laser bar of FIG. 1 mounted on a cooling slab 130 forming a modular unit. The laser diode emitters 102 are configured to emit the output beams as shown in a parallel orientation to the major dimension of the cooling slab 130. The cooling slab 130 is typically made of a material with a high heat conductance value, such as copper. Fluid passages 132, 134 are typically used to supply a coolant liquid such as water. Small fluid passages 136, 138 or microchannels may be included in the slab to facilitate heat removal from the diode bar.
Because laser diode bars have been limited in power output, even with cooling slabs as shown in FIG. 2, additional attempts have been made to increase the intensity and fluences of outputs from such diode bars. For example, laser bars have been placed adjacent to one another or stacked in stacked laser diode bar assemblies. One example of a stacked laser diode bar assembly is shown in FIG. 3.
FIG. 3 is a perspective view of a stack 300 of diode bar and cooling slab units shown in FIG. 2. Multiple diode bars 301 with emitters 302 are mounted in between cooling slabs 330 in a modular arrangement. Cooling passages or channels 332 and 334 may be included for liquid heat transfer. Spacers 340 may be present to facilitate alignment of the diode bars 301 and cooling slabs 330.
For certain applications, such as when used for optical excitation means or as a pump source, it may be desirable for a light source to produce light that has a high brightness. The brightness of a given light source, for example a laser diode or diode bar 301, is described by the brightness equation in Equation (1):B=P/(A*Ω)  (1)
In Equation 1, B is the brightness of the light source, P is the power output of the particular light source, Ω is the solid angle of the beam divergence, and A is the area of the light source. The brightness of a given light source consequently includes a power component, an area component and a divergence component. Typical units of measure are Watts for P, steradians (ster) for Ω, cm2 for A, and Watts/cm2/ster for B.
One limitation of the attainable brightness of stacked laser bar assemblies, such as 300, is the spacing or “pitch” between laser bars in the particular stack. According to Equation 1, the brightness of a laser diode bar and stacks of such diode bars is reduced by the percentage of non-light-emitting area of the structure outside the diode emitters. The ratio of the total emitter area compared to the total area of a stack is sometimes referred to as the fill factor of the diode stack.
With continued reference to FIG. 3, the non-light-emitting area of the diode bar stack 300 includes the space between emitters 302 in a particular diode bar and the space between adjacent diode bars. The space between adjacent diode bars, and therefore the fill factor, is primarily determined by the need to remove waste heat from the laser diodes. Without proper heat removal, the lifetime of the laser diode components are shortened, and wavelength fluctuations may occur over time. Currently, in systems that employ continuous wave (CW) diode bars, power densities about 200 W/cm2 can be achieved, but this power density is currently limited to this amount, since the diode bars cannot be placed much closer together than 1–3 mm, for a 60 W diode bar, due to thermal management concerns.
FIG. 4 is a perspective view of an alternate prior art diode bar stack 400 including a cooling slab 420. Each of the diode bars 402 includes a number of emitters 404. The diode bars 402 are separated by copper plates 406 that may act as heat spreaders and as electrical or ohmic contacts for the diode bars 402. The multiple diode bars 402 are mounted on the cooling slab 420 in an orientation where the diode outputs (not shown) are perpendicular to the cooling slab 420. A substrate is positioned between the diode bars and the cooling slab 420, and the substrate typically includes electrical conductors 408 and electrical insulators 410 to facilitate current flow through the active regions of the diode bars 402.
Prior art diode stacks such as those shown in FIG. 3 and FIG. 4 may have low fill factors when designed for high-power operation and consequently may produce a light output that has a relatively low brightness and/or fluence for the total area of the diode stack and for a given applied power.